I wonder how difficult this would be to achieve? A simple UI tweak to all displayed values, or something more involved? I've worked on things that natively use the metric system, but can readily display in imperial with ease, but then it was designed that way for the convenience of users world-wide (aka, US and everywhere else pretty much lol). IF this were indeed a fairly simple thing for FDev to do, why not, if I enhances a group of players (US) experience of the game? However, I suspect ED's ancient spaghetti code isn't quite so easy to work with, judging by other "simple" things that either took years to happen, or never happened.
In the UK, we're sort of a hybrid, being brought up on both systems. I.e. ask me how tall I am and I'll say 6'4", ask me measure something that's not me and I'll answer in Metres and Centimetres. Same with weight, I'll say I'm [redacted]Stone, 13lbs, yet other things I'm weighing in kilos. It's similar with distances. Everywhere here is signposted in MILES, not Kilometres. Car's speedo shows both - an EU compliance thing IIRC. However, I'm perfectly comfortable in Kilometers for other things. It's very confusing, but not for us, we're used to it being that way! lol. I don't know if kids in school now are still being taught both like I was.
So, yeah, why not add it if people want it and it's a nice quick win for FDev? Would I enable it? Nope, but I'm not a "you can't have it because I wouldn't use it" sort of person.